Error Correction Code-based Embedding in Adaptive Rate Communication Systems

ABSTRACT

The invention relates to concealing information within error correction codes of adaptive rate wireless communication systems. In some embodiments, the invention includes selecting a modulation and coding scheme with a more robust error correction capacity than needed by current channel conditions; encoding a hidden message with a pre-shared key that is known by a covert transmitter and a covert receiver, and after a standard message is encoded by a transmitting station of the wireless communication systems, replacing codeword parity bits of codewords in the encoded standard message with the encoded hidden message at designated locations. Before a receiving station of the wireless communication systems decodes the encoded standard message, a covert receiver extracts the embedded hidden message from the encoded standard message, replaces bit values of the embedded hidden message with zero at the designated locations, and decodes the extracted hidden message with the pre-shared key.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/964,533, filed Jan. 22, 2020, which is hereby incorporated in itsentirety by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to methods and systems forconcealing information in wireless communication systems.

2. Description of the Related Art

Information hiding techniques utilize legitimate carriers to transporthidden messages, providing users with some measure of anonymity andsecurity. Recently there has been increased attention to the role ofinformation hiding techniques with respect to cyber warfare and crime.The ability to perform command and control of malware, payload delivery,and recovery of desired content relies upon the development ofcommunication paths that evade cyber defenses. Covert channels will notonly obscure the content of these vital communication links, but furtherconfound efforts by computer security and forensic professionals bymaking the channels difficult to detect in the first place. The termcovert channel is defined as a channel that was “not intended forinformation transfer at all”; these channels are implemented usinginformation-hiding techniques.

A popular technique used to carry large hidden payloads is digital mediasteganography. Hidden data is carried within modifications made to acover object which are imperceptible to both unwitting users andpotential eavesdroppers. A substantial weakness of this technique isthat the selection of a cover object is restricted to items that cantolerate a certain measure of distortion (to include images, video, andaudio). Other objects commonly found in modern networked communications,including text or executable files, cannot be utilized as the act ofembedding the hidden message irreparably degrades the original content.

An alternative information hiding technique previously explored inliterature involves the use of forward error correction (FEC) codes.While FEC is commonly utilized to protect covert data from interference,they have also been used to carry hidden payloads. As discussed inrelevant papers, FEC is attractive for information hiding as these codesoften provide more redundancy than required by channel conditions; thisredundancy can be used to carry hidden data. In addition, most modemcommunication protocols also include retransmission mechanisms that canresend lost or corrupted data if the embedded FEC fails to correct allbit errors.

In recent years, wireless communications networks have grown torepresent the majority of all Internet traffic, and there has beenincreased interest in the development of information hiding techniquesthat exploit vulnerabilities within these systems. Against thisbackdrop, embodiments herein describe potential information hidingopportunities associated with a modern wireless local area network(WLAN) protocol, specifically the millimeter wave (mmWave) Institute ofElectrical and Electronics Engineers (IEEE) 802.11ad directionalmulti-Gigabit (DMG) standard. Specifically, the embodiments seek toleverage the legitimate mechanisms within the standard to supportinformation hiding techniques and develop a high-throughput covertchannel.

The IEEE 802.11ad DMG specification provides the medium access controland physical layer (PHY) amendments necessary to achieve extremely highdata rates (up to 8 Gbps) in one of six 2.16-GHz channels [16]. IEEE802.11ad is intended for use in high-bandwidth, short-range,line-of-sight (LOS) applications to include wireless cable replacementfor high definition video, wireless peripherals and docking stations, orother traditional WLAN implementations.

The standard was originally specified for three different PHY modulationmodes: the control PHY, single carrier (SC) PHY and orthogonalfrequency-division multiplexing (OFDM) PHY. Both the control and SC PHYare mandatory for all devices and there is also an optional low-power SCPHY defined for power-constrained devices. The OFDM PHY is nowconsidered obsolete and therefore the focus of embodiments herein is onthe SC PHY. Baseband processing at the transmitter for all PHY involvesa scrambler, encoder, modulator, and insertion of guard intervals (GI).A simplified block diagram of the transmit and receive process 100 isshown in FIG. 1A.

All modem high-capacity wireless fidelity (Wi-Fi) protocols rely upon apredetermined set of modulation and coding schemes (MCS) to facilitateefficient communication across a range of channel conditions. Theflexibility of MCS-based systems enable wireless networks to optimizethroughput while meeting required error thresholds. Another criticalaspect of MCS-based systems is the ability to perform link adaptation,where stations can dynamically select the optimal MCS for the currentchannel conditions. While link adaptation implementations vary, a commonmetric utilized to select the appropriate MCS is packet error ratio(PER). PER measures the number of packets that contain bit errors afterthe FEC decode process; the selection of the MCS by the link adaptationscheme is intended to maximize throughput while maintaining PER below aspecified limit.

In the IEEE 802.11-2016 standard, a revision to the original DMGspecification added a number of MCS indices to the SC PHY including a7/8-rate low-density parity-check (LDPC) code. This code rate wasachieved by passing data through the existing 13/16-rate encoder andthen puncturing the first 48 parity bits. Under normal operations withan MCS determined by the channel state, the capacity of a FEC-basedinformation hiding scheme is the shaded area 108 of FIG. 1B. FIG. 1Bshows the instantaneous channel capacity 106 and throughput with dynamicMCS 110.

SUMMARY OF THE INVENTION

Embodiments in accordance with the invention relate to a method andsystem for concealing information in wireless communication systems. Insome embodiments, the method includes selecting a modulation and codingscheme with a more robust error correction capacity than needed bycurrent channel conditions; encoding a hidden message with a pre-sharedkey that is known by a covert transmitter and a covert receiver; andafter a standard message is encoded by a transmitting station of thewireless communication systems, replacing codeword parity bits ofcodewords in the encoded standard message with the encoded hiddenmessage at designated locations. Before a receiving station of thewireless communication systems decodes the encoded standard message, acovert receiver extracts the embedded hidden message from the encodedstandard message, replaces bit values of the embedded hidden messagewith zero at the designated locations, and decodes the extracted hiddenmessage with the pre-shared key.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a simplified block diagram of a typical transmit andreceive process.

FIG. 1B shows a covert channel capacity of an MCS under normaloperation.

FIG. 2A shows throughput of a communication system with lower MCSintentionally selected.

FIG. 2B shows cover channel capacity with a lower MCS selected.

FIG. 3 shows a system workflow for a cover channel in accordance withembodiments described herein.

FIG. 4A shows an FEC-protected hidden message using a standard embeddingtechnique.

FIG. 4B shows an FEC-protected hidden message using embedding techniqueaccording to embodiments described herein.

Embodiments in accordance with the invention are further describedherein with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is provided to enable any person skilled inthe art to use the invention and sets forth the best mode contemplatedby the inventor for carrying out the invention. Various modifications,however, will remain readily apparent to those skilled in the art, sincethe principles of the present invention are defined herein specificallyto concealing information in wireless communication systems.

It is theorized that if an SC DMG system was operating under channelconditions that supported a 7/8-rate code MCS (i.e., MCS 9.1), the 48parity bits that would normally be punctured might be able to carry ahidden payload if an MCS with the 13/16-rate code (i.e. MCS 9) wasintentionally selected. Embodiments herein described a FEC-basedinformation hiding technique that increases covert channel capacity byleveraging the MCS construct and link adaptation functionality. Theincreased capacity 212 is visualized in FIGS. 2A and 2B. Attempts toincrease the throughput 210 of the information hiding scheme can exceedthe instantaneous channel capacity 206 and result in system data beingdelivered with an increased probability of error. FIG. 2A shows thechange in communication channel performance resulting from theintentional selection of a lower MCS index 210; which enables increasedcovert channel capacity 212, shown in FIG. 2B, without causing increasederror rates to the underlying data.

There are two assumptions that should be made to realize the covertchannel described herein. First it is assumed that covert access hadbeen gained to both the transmitting and receiving station to facilitateimplementation of the information hiding scheme. Second, while bothstations will require software, firmware, or hardware modifications, itcannot be assumed that duplex communication is possible. As a result,some of the mechanisms utilized for normal error correction andredundancy, including automatic repeat request, may not be available tosupport the covert channel; other methods must be used to ensure theredundancy of the hidden data.

Embodiments herein describe a forward error correction-based informationhiding technique for adaptive rate wireless communication systems.Specifically, the functionality of wireless local area networkmodulation and coding schemes (MCS) and link adaptation mechanisms areleveraged to significantly increase covert channel throughput. Below isa detailed implementation of this technique within the IEEE 802.11ad,directional multi-Gigabit standard. Simulation results demonstrate thepotential of the proposed techniques to develop reliable,high-throughput covert channels under multiple MCS rates and embeddingtechniques. Covert channel performance is evaluated in terms of theobserved packet error ratio of the underlying communication system aswell as the bit error ratio of the hidden data.

FIG. 3 shows a flowchart 300 for concealing information in wirelesscommunication systems. As is the case with this and other flowchartsdescribed herein, various embodiments may not include all of the stepsdescribed below, may include additional steps, and may sequence thesteps differently. Accordingly, the specific arrangement of steps shownin FIG. 3 should not be construed as limiting the scope of concealinginformation in wireless communication systems.

A functionality of the proposed MCS-based information hiding techniquedepends upon the ability to embed hidden data within the parity bits ofthe FEC codeword. The amount of hidden data accommodated by this schemecan be increased by selecting a lower MCS-index as described above.Risks of detection include lower than expected system throughput givencurrent channel conditions and the potential for an increased number ofuncorrectable packet errors. Embodiments described herein minimize bothrisks.

Transmission of legitimate data input 302 is initially scrambled 304 andencoded 306 at a transmitting station. Data embedding 314 by a coverttransmitter occurs at baseband at after the LDPC encoder 306. Theembedding locations 314 are coordinated between the covert transmitterand covert receiver through a pre-shared key; analogous to the methodused to implement the 7/8-rate puncturing scheme, the hidden data 308 isencoded 312 and embedded 314 in the first n parity bits of eachcodeword. Once the embedding process is complete, the modified LDPCcodeword is passed to the modulator 316 before completing the rest ofthe transmit process (blocks 318-324).

At the destination, the hidden message is recovered by the covertreceiver at 326 after demodulation 324. The output of the receivingstation demodulator 324 are log-likelihood ratio (LLR) values 326; LLRvalues 326 represent both a bit value and a confidence level. Afterdecoding 328 and extraction 332 of the embedded message, {circumflexover (v)}, the bit positions that carried the hidden data are assigned avalue of 0 before being sent to the LDPC decoder at 334 by the covertreceiver. In the IEEE Standard it specifies that for punctured codes,LLR values of 0 are used at the decoder 336 to prevent the stuffed bitsfrom introducing additional error; this same principle is leveraged toprevent the LLR values of the embedded hidden data from corrupting thelegitimate packet payload. The legitimate packet payload is thendescrambled 338 by the receiving station to obtain the legitimate dataoutput 340.

Experimental trials of this proposed technique were conducted in MATLAB.The simulation was adapted from a MATLAB-developed script to measurePER; embedding and extraction of the hidden message requiredmodifications to existing encode and decode functions used within theMATLAB WLAN Toolbox. The performance of the proposed technique wasassessed by measuring the PER of the underlying communication system aswell as the bit error ratio (BER) of the hidden data. PER identified thenumber of uncorrectable packet errors experienced at a specified channelcondition and embedding rate; BER provided an estimate of expectederrors in the received hidden data. As noted above, it was not assumedthat duplex communication would be available in the covert channel andtherefore was highly desirable to minimize the BER of the receivedhidden data.

The simulation was based around the transmission of a single packet witha 4096 octet PHY service data unit (PSDU). This PSDU length was selectedbased on criteria outlined in the IEEE Standard, which specified thatthe PER for each SC MCS index be no more than 1% given a PSDU length of4096 octets. Additive white Gaussian noise (AWGN) was utilized tosimulate interference in the channel. A consistent seed value was usedfor AWGN generation to ensure the noise environment remained consistent,with the only variable being the amount of hidden data embedded percodeword. While only a single packet was sent between the transmittingand receiving stations, these trials were completed multiple times todevelop an accurate representation for the PER and BER at each specifiedsignal-to-noise ratio (SNR) value; up to 10000 trials were conducted ateach SNR value to evaluate MCS and embedding combinations.

The first series of MATLAB simulations served as a proof of concept ofthe general information hiding technique and were designed to addressthree main objectives. First, could a hidden message, m, be successfullyembedded and extracted from a designated block of parity bits. Second,determine the upper limit of embedding, measured in bits-per-codeword,which would result in a noticeable performance degradation to theunderlying overt communication system. Finally, could the receivedhidden message, {circumflex over (m)} be reliably estimated given thepresence of channel noise. Initial simulations using MCS 9 wereconducted under channel-conditions that would support MCS 9.1. Thesimulated transmitter replaced the first 48 parity bits of each13/16-rate LDPC codeword with hidden data; the hidden data was not FECprotected. Additional trials were then conducted to determine thebehavior of the covert channel and underlying communication system atdifferent levels of embedding. The capacity of the system is impacted bythis technique; when MCS 9 is selected in lieu of MCS 9.1, the maximumthroughput is reduced from 2695 Mbps to 2502.5 Mbps. The results fromvarious embedding rate trials were considered where the PER for theembedding rates is compared to that of the baseline MCS index; asexpected the PER of MCS 9, at an embedding rate of 48 bits-per-codeword,is equivalent to that of baseline MCS 9.1. The embedded data duringthese initial trials was uncoded and therefore did not benefit from anyerror correction. As a result, the BER of the received hidden data, A,was consistent with the probability of bit error of uncoded quadraturephase-shift keying (QPSK):

$\begin{matrix}{{P_{b} = {Q\left( \sqrt{\frac{2E_{b}}{N_{o}}} \right)}},} & (1)\end{matrix}$

where E_(b) is the energy per user data bit, and N is the noise spectraldensity.

The performance characteristics of the covert channel was relativelypredictable when subjected to varying levels of embedding activity. ThePER of the underlying communication system increased up to the limit of48 bits-per-codeword, with embedding rates of 24 bits-per-codewordrequiring a lower SNR to achieve a given PER than the 36bits-per-codeword embedding rate, but a higher SNR than the 12bits-per-codeword rate.

Simulations were then conducted for the IEEE 802.11ad MCS indices thatutilized QPSK modulation (MCS 6-9.1); embedding capacity estimates aredisplayed below in TABLE I. The maximum number of bits-per-codeword foreach 4096 octet PSDU was determined experimentally for each MCS. Theseresults were used to calculate a ratio for embedding capacity and thenmultiplied by the data rates published in the IEEE Standard to estimatethe covert channel throughput at each MCS.

TABLE I MCS Dictated MCS Used Max by Channel by Embedding EmbeddingConditions Algorithm Coding Scheme per CW MCS 7 MCS 6 LDPC (1/2) 92 bitsMCS 8 MCS 7 LDPC (5/8) 85 bits MCS 9 MCS 8 LDPC (3/4) 48 bits MCS 9.1MCS 9 LDPC (13/16) 48 bits Embedded Embedded Est. Covert Bits per 4096Bits as % of Channel octet PSDU Data Rate Throughput 9016 bits 27.51%423.73 Mb/s 6715 bits 20.49% 394.48 Mb/s 3168 bits 9.67% 223.33 Mb/s2928 bits 8.94% 223.46 Mb/s

Although the lack of FEC in these trials maximized the embeddingcapacity of the information hiding technique, the relatively high BERwould have made maintaining reliable covert communications difficult.

After the initial trials, LDPC codes from the IEEE 802.1 ladspecification were applied to the covert channel by parsing the hiddenmessage into codewords. While the FEC codes utilized for this task wereidentical to those defined in the IEEE Standard, rate-selection wasinfluenced by the observed channel condition and not necessarily thediminished code-rate utilized for the underlying communication channel.Based on the maximum embedding rates determined in the initial trials,it was necessary to calculate the number of FEC-protected hidden messagebits, K, that could be embedded in each PSDU. First, the number of LDPCcodewords in each PSDU, N_(CW), was calculated according to the IEEEstandard:

$\begin{matrix}{{N_{CW} = \left\lceil \frac{8\rho \; L_{P}}{L_{CW}R_{C}} \right\rceil},} & (2)\end{matrix}$

where L_(P) is the length of the PSDU (in octets). The remainingvariables are all dependent on the selected MCS, with L_(CW) being thelength of the LDPC codeword, R_(C) being the LDPC code rate, and p beingthe repetition factor of the code.

In order to determine the amount of FEC-protected hidden data that canbe embedded into a given PSDU, it is then necessary to determine theamount of data in each LDPC codeword used to carry the hidden message,D_(ECW), where

D _(ECW) =L _(ECW) R _(EC),  (3)

given the length of the embedded data codeword, L_(ECW), and the rate ofthe FEC code used to protect the hidden message, R_(EC). Using theresults from (1) and (2), it is possible to determine, K, the number ofFEC-protected hidden message bits that can be embedded given the maximumnumber of embedded bits-per-PSDU codeword, n:

$\begin{matrix}{K = {D_{ECW}{\left\lfloor \frac{{nN}_{CW}}{L_{ECW}} \right\rfloor.}}} & (4)\end{matrix}$

Utilizing FEC to protect the hidden data reduced the capacity of theinformation hiding scheme. Capacity was lost due to the need to embedFEC parity bits and the fact that only complete hidden message FECcodewords could be embedded in the PSDU. For trials at MCS 9, themaximum embedding capacity was 48 bits-per-codeword during the initialtrials, or 2928 bits-per-PSDU. With FEC applied, only four, 624-bitcodewords could be embedded, leaving 240 bits unaltered. The use of FECin MCS 9 limited the capacity of each 4096 octet PSDU to 2184 embeddedbits. Although capacity was reduced, FEC significantly improved thehidden data BER. The SC DMG specification calls for a minimum PER of

10⁻², or 1% (IEEE Standard); a 1% PER is achieved at an SNR ofapproximately 8.6 dB. At 8.6 dB the BER for the FEC-protected hiddendata is approximately 7.8×10⁻⁷. As before, trials were then conductedfor all QPSK-based MCS with embedding capacity estimates provided belowin TABLE II.

TABLE II MCS Dictated MCS Used Coding Utilized by Channel by EmbeddingCoding Utilized for Embedded Conditions Algorithm for Overt Data DataMCS 7 MCS 6 LDPC (1/2) LDPC (5/8) MCS 8 MCS 7 LDPC (5/8) LDPC (3/4) MCS9 MCS 8 LDPC (3/4) LDPC (13/16) MCS 9.1 MCS 9 LDPC (13/16) LDPC (7/8)Embedded Embedded Est. Covert Bits per 4096 Bits as % of Channel octetPSDU Data Rate Throughput 5460 bits 16.67% 256.60 Mb/s 4536 bits 13.84%266.47 Mb/s 2184 bits 6.67% 153.96 Mb/s 2184 bits 6.67% 166.79 Mb/s

Finally, a modified embedding technique was developed with the aim ofreducing the distortion on the underlying communications channel withoutsacrificing the throughput of the covert channel. In the originalapplication of FEC, the encoded embedded message bits were inserted intothe parity bits such that the full n bit embedding capacity was utilizedin the first f codewords, before embedding the remainder r_(s) bits ofhidden data in the codeword in position (f+1). A representation 402 ofthis method is shown in FIG. 4A with the number of fully embeddedcodewords, f; calculated as:

$\begin{matrix}{{f = \left\lfloor \frac{T}{n} \right\rfloor},} & (5)\end{matrix}$

where T is the total number of bits to be embedded. The number of bitsembedded in the final codeword that contains hidden data, r_(s), isdetermined using the following formula:

$\begin{matrix}{r_{s} = {{T - {nf}} = {T - {n{\left\lfloor \frac{T}{n} \right\rfloor.}}}}} & (6)\end{matrix}$

Since the total number of parity bit locations available for embeddingwas greater than the number of bits being embedded, this method resultedin some legitimate codewords having all n parity bits embedded, whileothers carried 0 embedded bits. Since codewords that were fully embeddedare more likely to experience an uncorrectable error, and that anuncorrectable error in any codeword would result in a packet error, analternative embedding process was developed.

Embodiments herein can utilize a process similar to interleaving wherethe hidden data is distributed equally across all N_(CW) codewords. Thismethod 404 is illustrated in FIG. 4B where the total number of codewordsw, is equal to N_(CW). Every codeword was embedded with a minimum of pbits:

$\begin{matrix}{{p = \left\lfloor \frac{T}{w} \right\rfloor},} & (7)\end{matrix}$

where T remains the total number of bits being embedded. While allcodewords contain at least p-bits, the first r_(i) codewords willcontain (p+1) bits; r_(i) is calculated by:

$\begin{matrix}{{r_{i} = {{T - {wp}} = {T - {w\left\lfloor \frac{T}{w} \right\rfloor}}}},} & (8)\end{matrix}$

The final series of experimental trials were conducted utilizing thisinterleaving technique. While the modification did not result in anincrease in embedding capacity, it did consistently reduce the impact ofthe embedding on the underlying communication system. This improvementallows for 2184 message bits to be embedded in each 4096 octet PSDU witha 0.25 dB reduction in the required SNR. The embedded data BER for theinterleaved case remains unchanged from the standard FEC implementation.

This improved performance persists across the previously considered MCSindex values that utilize QPSK modulation (MCS 6-9.1). A summary of theSNR performance of this final iteration, along with the associated BERfor the embedded data is shown below in TABLE Ill.

TABLE III MCS Dictated by MCS Used by Channel Conditions EmbeddingAlgorithm MCS 7 MCS 6 MCS 8 MCS 7 MCS 9 MCS 8 MCS 9.1 MCS 9 SNR at 1%PER with SNR at 1% MCS Dictated PER with Embedded Embedded by ChannelInterleaved Bits per 4096 Data BER Conditions Embedding octet PSDU (at1% PER) 5.07 dB 4.91 dB 5460 bits 1.3 × 10⁻⁶ 6.41 dB 6.26 dB 4536 bits5.4 × 10⁻⁶ 7.47 dB 7.21 dB 2184 bits 4.1 × 10⁻⁶ 8.61 dB 8.31 dB 2184bits 7.8 × 10⁻⁷

Embodiments herein provide a high-throughput covert channel, more than150 Mbps, that reliably delivers a hidden payload without significantlyincreasing the errors observed within the underlying communicationssystem. While this detailed description focuses on IEEE 802.11ad,leveraging MCS selection to increase covert channel capacity is alsoapplicable to other adaptive rate communication protocols.

As noted in the initial simulations, the behavior of the informationhiding technique was predictable when subjected to varying levels ofembedding. This characteristic may allow the use of variable rateembedding without decrementing the MCS; by sensing the channel anddetermining if the channel state exceeds the minimum requirements of thecurrent MCS index, embedding rates could be dynamically selected tomaximize covert channel capacity while minimizing distortion.

The invention may be implemented on virtually any type of computerregardless of the platform being used. For example, a computer systemcan include a processor, associated memory, a storage device, andnumerous other elements and functionalities typical of today'scomputers. The computer may also include input means, such as mixedreality controllers or a keyboard and a mouse, and output means, such asa display or monitor. The computer system may be connected to a localarea network (LAN) or a wide area network (e.g., the Internet) via anetwork interface connection. Those skilled in the art will appreciatethat these input and output means may take other forms.

Further, those skilled in the art will appreciate that one or moreelements of the computer system may be located at a remote location andconnected to the other elements over a network. Further, the inventionmay be implemented on a distributed system having several nodes, whereeach portion of the invention may be located on a different node withinthe distributed system. In one embodiment of the invention, the nodecorresponds to a computer system. Alternatively, the node may correspondto a processor with associated physical memory. The node mayalternatively correspond to a processor with shared memory and/orresources. Further, software instructions to perform embodiments of theinvention may be stored on a computer readable medium such as a compactdisc (CD), a diskette, a tape, a file, or any other computer readablestorage device.

This disclosure provides exemplary embodiments of the present invention.The scope of the present invention is not limited by these exemplaryembodiments. Numerous variations, whether explicitly provided for by thespecification or implied by the specification or not, may be implementedby one of skill in the art in view of this disclosure.

What is claimed is:
 1. A method for concealing information within errorcorrection codes of adaptive rate wireless communication systems, themethod comprising: selecting a modulation and coding scheme with a morerobust error correction capacity than needed by current channelconditions to increase a covert channel capacity of the wirelesscommunication systems; encoding a hidden message with a pre-shared keythat is known by a covert transmitter and a covert receiver; and after astandard message is encoded by a transmitting station of the wirelesscommunication systems, replacing codeword parity bits of codewords inthe encoded standard message with the encoded hidden message atdesignated locations; wherein before a receiving station of the wirelesscommunication systems decodes the encoded standard message, a covertreceiver: extracts the embedded hidden message from the encoded standardmessage; replaces bit values of the embedded hidden message with zero atthe designated locations; and decodes the extracted hidden message withthe pre-shared key.
 2. The method of claim 1, wherein the codewordparity bits are a first n parity bits of each codeword of the encodedstandard message.
 3. The method of claim 2, wherein the bit values arelog-likelihood ratio values.
 4. The method of claim 2, wherein thecodewords are forward error correction codewords.
 5. The method of claim1, wherein the parity-check code is a low-density parity-check code. 6.The method of claim 1, wherein the codeword parity bits for eachcodeword is a minimum of p bits according to:${p = \left\lfloor \frac{T}{w} \right\rfloor},$ wherein T is a totalnumber of bits being embedded in the standard message and w is a totalcount of the codewords in the standard message, and wherein eachcodeword does not include an uncorrectable error.
 7. The method of claim1, wherein the adaptive rate wireless communication systems support a802.11ad protocol.
 8. A non-transitory computer-readable mediumcomprising executable instructions for causing a computer processor to:selecting a modulation and coding scheme with a more robust errorcorrection capacity than needed by current channel conditions toincrease a covert channel capacity of adaptive rate wirelesscommunication systems; encode a hidden message with a pre-shared keythat is known by a covert transmitter and a covert receiver; and after astandard message is encoded by a transmitting station of the wirelesscommunication systems, replace codeword parity bits of codewords in theencoded standard message with the encoded hidden message at designatedlocations; wherein before a receiving station of the wirelesscommunication systems decodes the encoded standard message, a covertreceiver: extracts the embedded hidden message from the encoded standardmessage; replaces bit values of the embedded hidden message with zero atthe designated locations; and decodes the extracted hidden message withthe pre-shared key.
 9. The non-transitory computer-readable medium ofclaim 8, wherein the codeword parity bits are a first n parity bits ofeach codeword of the encoded standard message.
 10. The non-transitorycomputer-readable medium of claim 9, wherein the bit values arelog-likelihood ratio values.
 11. The non-transitory computer-readablemedium of claim 9, wherein the codewords are forward error correctioncodewords.
 12. The non-transitory computer-readable medium of claim 8,wherein the parity-check code is a low-density parity-check code. 13.The non-transitory computer-readable medium of claim 8, wherein thecodeword parity bits for each codeword is a minimum of p bits accordingto: ${p = \left\lfloor \frac{T}{w} \right\rfloor},$ wherein T is a totalnumber of bits being embedded in the standard message and w is a totalcount of the codewords in the standard message, and wherein eachcodeword does not include an uncorrectable error.
 14. The non-transitorycomputer-readable medium of claim 8, wherein the adaptive rate wirelesscommunication systems support a 802.11ad protocol.